There are certain types of fast couplings which consist of a threaded pipe (threaded in one or both of its ends) so that it allows a nut to make a connection to a second pipe. The second pipe is inserted into the opening of the threaded pipe and a toroidal element of rubber is pressed between the second pipe and the inside wall of the nut from the pressure exerted by the nut.
This union can only be used in installations where the elements to be fitted are secured to other supporting structures, because if this is not done, the line pressure can generate an axial shifting of the pipes.
Also well known is an assembly which consists of a tubular body that has, near one of its ends, a threaded ring section that terminates in a frustum cone so that its bigger diameter is slightly bigger than the inside diameter of the pipe to be fitted and its smaller diameter is slightly smaller than the inside diameter of the pipe. The union is complemented with a junction nut which consists of an internally threaded body which is partially blocked on one of its ends by a flap or side wall, which has in its center a circular opening to allow the inserting or tight fitting of the pipe to be connected. The coupling is made by introducing the frustum cone end portion of the assembly into a pipe end, which in turn widens the flexible pipe up to the threaded ring so that the pipe is permanently subjected to stress. This set is fixed by the flapped nut to prevent accidental axial movement of the pipe.
In this type of assemblies, the pipes are stressed towards the releasing of the pipes in the direction of the axial component of a force. This stress component is a factor of instability when the nut is not safely secured.
It is necessary to have a secure coupling even when the nut is noticeably loose.